1. Technical Field
The present disclosure relates to a switching power supply circuit and a method for driving a switching power supply circuit.
2. Description of Related Art
Switching power supply circuits, exhibiting superior linear characteristics, consuming minimum power, converting electrical power efficiently, and performing steadily, are preferred for use in LCD TVs, displays, and other consumer devices.
FIG. 3 shows a commonly used switching power supply circuit. The switching power supply circuit 10 includes a first rectifying and filtering circuit 11, a snubber circuit 13, a transformer 14, a second rectifying and filtering circuit 12, a switching circuit 15, and a rectifying diode 16. The switching circuit 15 includes a pulse width modulation integrated circuit (PWM IC) 151, a transistor 152, and a resistor 155. The PWM IC 151 includes a voltage input 153 for receiving an operating voltage, and a pulse output 154 for transmitting a generated pulse signal to a gate electrode of the transistor 152.
The first rectifying and filtering circuit 11 includes two inputs 113, 114 structured and arranged to receive an external alternating current (AC) voltage such as a 220V AC voltage, a full-bridge rectifying circuit 111 structured and arranged to convert the 220V AC voltage to a first direct current (DC) voltage, a first filtering capacitor 112 structured and arranged to stabilize the first DC voltage, and a first output 115 structured and arranged to provide the first DC voltage to the transformer 14. Two inputs of the full-bridge rectifying circuit 111 serve as the two inputs 113, 114. A positive output of the full-bridge rectifying circuit 111 serves as the first output 115. A negative output of the full-bridge rectifying circuit 111 is grounded. The first filtering capacitor 112 is connected between the first output 115 and ground.
The transformer 14 includes a primary winding 141, an auxiliary winding 142, and a secondary winding 143. The primary winding 141 is electrically connected in parallel with the snubber circuit 13. One terminal “a” of the primary winding 141 is connected to the first output 115, and the other terminal “b” of the primary winding 141 is connected to a drain electrode of the transistor 152. A source electrode of the transistor 152 is grounded via the resistor 155. The gate electrode of the transistor 152 is connected to the pulse output 154 of the PWM IC 151.
One terminal of the auxiliary winding 142 is grounded. The other terminal of the auxiliary winding 142 is connected to the voltage input 153 of the PWM IC 151 via an anode and a cathode of the rectifying diode 16 in series.
The second rectifying and filtering circuit 12 includes an inductor 121, a storage capacitor 122, a second filtering capacitor 123, a rectifying diode 124, and a second output 125. One terminal “c” of the secondary winding 143 is connected to the second output 125 via an anode and a cathode of the rectifying diode 124 and the inductor 121 in series. The other terminal “d” of the secondary winding 143 is grounded. The second filtering capacitor 123 is connected between the second output 125 and ground. The storage capacitor 122 is connected between the cathode of the rectifying diode 124 and ground.
The switching power supply circuit 10 operates as follows:
The external AC voltage is provided to the two inputs 113, 114 of the first rectifying and filtering circuit 11, and is converted to the first DC voltage by the first rectifying and filtering circuit 11. The first DC voltage is provided to the primary winding 141. The auxiliary winding 142 is induced by the primary winding 141, generates an operating voltage, and provides the operating voltage to the voltage input 153 of the PWM IC 151 via the rectifying diode 16. Thus the PWM IC 151 generates the pulse signal for switching the transistor 152 on or off.
When the transistor 152 is switched on, a first current path is formed sequentially through the first output 115, the primary winding 141, the transistor 152, and the resistor 155. A first current I is formed when the first DC voltage provided to the first output 115 is connected to ground via the first current path. The first current I through the first current path increases linearly until electromagnetic induction generated in the primary winding 141 reaches a predetermined maximum threshold. Thus a voltage across the primary winding 141 is generated such that a potential of the terminal “a” is higher than that of the terminal “b”. The secondary winding 143 is induced by the increasing first current I to generate a first induced voltage such that a potential of the terminal “c” is higher than that of the terminal “d”. The rectifying diode 124 and the inductor 121 convert the first induced voltage to a second DC voltage, and provide the second DC voltage to a load circuit (not shown) via the second output 125.
When the transistor 152 is switched off, the snubber circuit 13 gradually absorbs and decreases the first current I. The secondary winding 143 is induced by the decreasing first current I to generate a second induced voltage such that the potential of the terminal “d” is higher than that of the terminal “c”. Thus the rectifying diode 124 is switched off, and no voltage is outputted from the second output 125.
The switching power supply circuit 10 does not utilize the decreasing first current I to generate the second DC voltage when the transistor 152 is switched off and the first current I is absorbed by the snubber circuit 13. Thus, the switching power supply circuit 10 lacks efficiency.
It is thus desirable to provide a switching power supply circuit which can overcome the described limitations.